1. Field of the Invention
This invention relates to an image forming lens comprising a stop and a single lens, particularly an image forming lens to be used in an imaging apparatus containing such an imaging element as CCD or CMOS, and also relates to a portable information terminal having this image forming lens.
2. Description of the Related Art
In the multi-media field in recent years, electronic cameras and cell phones respectively containing an imaging lens are highly required to be extremely smaller in size, lighter in weight and lower in cost. So the image forming lens is also desired to be extremely smaller in size, lighter in weight, lower in cost and higher in resolution.
For achieving a lower cost, smaller size and lighter weight, an image forming lens consisting of a single lens is advantageous, but it is difficult to correct various aberrations. Especially the lens has the problem that the performance in the circumference of the lens is likely to deteriorate easily. However, various image forming lenses respectively consisting of a single lens have been proposed. One of the proposed image forming lenses is described in Patent Document 1 (JP2006-10990A).
The image forming lens of the invention described in Patent Document 1 consists of a stop and a single lens having paraxial curvature radii to form biconvex surfaces, which are placed in this order from the object side. If the object-side surface of said single lens is referred to as the first surface while the image-side surface is referred to as the second surface, both said first surface and said second surface have an aspherical shape, and the paraxial curvature radius of the first surface, the thickness of the lens, the distance between the first surface to the image forming face in terms of air length, the focal length, the distance from the stop to the first surface, Abbe constant, etc. are set to meet predetermined conditions. It is insisted that if this constitution is employed, an image forming lens of low cost, small size, light weight, good telecentricity and high performance suitable for an electrophotographic apparatus can be produced.
[Patent document 1] JP2006-10990A
However, the image forming lens described in said Patent Document 1 has the disadvantage that since it is intended to use such a resin as a norbornene-based polymer or polymethyl methacrylate in addition to glass as the material constituting the lens, the lens cannot be used in an imaging apparatus requiring heat resistance at about 250° C.
Moreover, the lens is required to be further improved in the correction of chromatic aberration, which is an inevitable problem with a single lens.
The present invention has been made in view of the above-mentioned situation. The object of this invention is to provide an image forming lens and a portable information terminal that are lower in cost, smaller in size and lighter in weight, and yet shortened in the overall optical length especially including the mechanical parts such as stop, good in telecentricity on the image face side, well corrected in chromatic aberration, high in performance and excellent in heat resistance.